Project Summary Immune defense against bacterial infection requires activation of conserved signaling pathways that upregulate production of inflammatory mediators to clear infection. Many pathogens, including the pathogenic Yersinia inhibit these signaling pathways in order to evade host immune defenses. Yersinia injects a virulence factor, YopJ, which blocks NF-?B and MAPK signaling. How immune defense is mediated against pathogens that block immune signaling pathways remains poorly understood. NF-?B blockade in macrophages exposed to bacterial PAMPs leads to cell death with characteristics of apoptosis, yet this death has pro-inflammatory consequences. Notably, increasing cytotoxicity of Y. pseudotuberculosis or Y. pestis results in decreased bacterial virulence, suggesting that induction of cell death in response to Yersinia serves as a host immune protective mechanism. Our central hypothesis is that cell death triggered in response to Yersinia blockade of NF-?B and MAPK releases pro-inflammatory signals that alert uninfected neighboring cells to the presence of infection. However, the cellular and molecular basis for this response remains unclear. Our recently published and preliminary data demonstrate that RIPK1 kinase activity is required for Yersinia-induced cell death. Moreover, RIPK1 kinase activity contributes to control of Yersinia infection and to inflammatory cytokine production in vivo. Nevertheless, how RIPK1 kinase activity and cell death are coupled to inflammatory responses and host defense against bacterial infection is not known. This is an important problem as this pathway likely responds to many pathogens that block critical innate immune signaling pathways and in the context of pathological stimuli that lead to RIPK1-induced cell death. We propose two Specific Aims to address this important gap in our knowledge. First we will define the cellular population that requires RIPK1 kinase activity, and determine whether RIPK1 functions in a cell-intrinsic or extrinsic manner to mediate anti-bacterial immune defense. Second, we will will determine the contribution of RIPK1 to downstream pathogen-specific immune responses and will dissect whether RIPK1 functions to control bacterial dissemination or replication.